1. Technical Field
The present invention generally relates to a distance measuring system and, more particularly, to a distance measuring system utilizing the propagation of radio frequency (RF) signals between remotely located RF communication units, even if non-line-of-sight measurements are required.
2. Description of the Related Art
There are many applications in which it is desirable to know a distance between two points. For example, an operator of a boat may desire to know the distance between the boat and an underwater obstacle as marked by a buoy. A surveyor makes a series of distance measurements to mark out a property line. A golfer may desire to know the distance remaining to the green or to some hazard on the hole being played. In the case of the golfer, this information may be obtained by reference to yardage markers on the golf course or by reference to so-called yardage books which provide the distances based on the distance from particular trees, sand traps, or other marker. However, since distance measurements to a green are generally relative to the center of the green, the distance may not be accurate for a particular pin placement on a given day. Also, if a player's ball is displaced from one of the markers, it is necessary to estimate the distance from the ball to the marker, thereby introducing further error into the distance determination. In addition, yardage markers and yardage books frequently fail to provide meaningful indications of the distance to a particular hazard.
U.S. Pat. No. 4,751,689 to Kobayashi discloses a method of measuring a distance on a golf course by determining a time difference between a radio-wave and a sonic-wave from a measuring point to a point to be measured followed by the multiplication of the time difference with the sonic velocity.
U.S. Pat. No. 4,136,394 to Jones includes a remote unit which emits a radio frequency pulse toward a base unit at the pin. The base unit upon receiving the pulse from the remote unit emits an acoustic signal toward the remote unit. Upon receipt of the ultrasonic signal, the remote unit measures the time interval between the sending of the RF pulse and receipt of the ultrasonic signal. From this time interval, the remote unit determines the distance between the remote unit and the base unit based on the speed of the ultrasonic signal through the air.
Such systems, however, suffer from several deficiencies. For example, the speed of sound varies in accordance with environmental factors such as temperature and humidity, thereby introducing uncertainties into the distance measurement. In addition, the operation of such systems can be degraded if objects are located between the points whose separation is to be measured.